Buy it in a dungeon's Kecleon shop. Be prepared with 1000 bucks.
kecleon shop
wonder mail
Impossible: evolve pikachu with a thunderstone
In most Pokemon Mystery Dungeon games, you must clear the main plot of the game. You will then unlock a special evolution area, such as the Luminous Cave in Mystery Dungeon Red or Blue or the Luminous Springs in Mystery Dungeon Time, Darkness or Sky. To evolve Pikachu, you will need to have a Thunderstone in your possession. When asked if your Pokemon needs an item to evolve, say yes and select the Thunderstone. If you are trying to evolve your player or partner Pikachu in Mystery Dungeon Time, Darkness or Sky, you will also need to complete the post-credits plot and recruit Palkia for you to be able to evolve. You've got to get a Thunderstone and when Pikachu reaches a certain level, take it to the Luminous Spring while it is holding the Thunderstone and it appears on the list. Choose Pikachu, and it should evolve. I don't know where you get a Thunderstone.
I'm pretty sure you can get Thunderstones at Lightning Field on the last few floors.
No, there is no requirement to use a Thunderstone in Pokémon Mystery Dungeon Explorers of Darkness. It is an optional item that can be used to evolve certain Electric-type Pokémon.
Pikachu doesn't evolve by level, you have to evolve him with a thunderstone.
Pokemon cannot be traded in the Mystery Dungeon series.
No.
In all the other Pokemon games you need a Thunderstone. So try getting one and going to luminous spring.
NO
Yes, but only in Pokemon Mystery Dungeon: Explorers of Sky.